Gyromultipliers of the Fifth Cyclotron Harmonic Based on High-Power Gyrotrons for Plasma Applications

We study theoretically the possibility of creating sources of terahertz radiation of high (up to several watts) power based on excitation of the fifth cyclotron harmonic in the frequency multiplication regime in high-power gyrotrons intended for plasma applications. It was previously shown that effective excitation of isolated (s = 4n + 1, n = 1, 2, 3, . . . ) cyclotron harmonics in gyrotrons is due to the specific property of the eigenmodes of cylindrical waveguides, according to which the conditions for simultaneous resonance of an electron beam with two TE modes having asymptotically multiple cutoff frequencies can be fulfilled. For the fifth cyclotron harmonic, this method was tested in experiments with a low-frequency kilowatt power level gyrotron. The use of gyrotrons for plasma applications will significantly increase the power and frequency of radiation based on the observed effect. To suppress spurious oscillation at the main cyclotron resonance, which occurs in the region of optimal magnetic fields for the multiplication effect, it is proposed to use frequency locking of the gyrotron by an external signal. Simulations performed in this work based on an averaged self-consistent gyrotron model shows the possibility of oscillation in the described radiation scheme with a power of several watts at a frequency of 1.25 THz with radiation at the fifth harmonic of the gyrofrequency in a recently developed sub-MW/0.25 THz gyrotron with TE_{19, 8} operating mode.